In human and veterinary medicine, considerable importance is attached to in vivo determination of blood flow rate because a sufficient flow rate is critical for vitality. In particular, the blood flow rate may be valuable for the detection of life-threatening stimulation states of the heart, and thus for controlling devices for assisting in the cardiovascular function, such as cardiac pacemakers or defibrillators.
Numerous sensors for the determination of flow rates are known from the art which are based on various measurement principles. These include, for example, sensors based on the Venturi or Bernoulli principle or the principle of Prandtl's pitot tube, as well as Doppler sonographic measuring systems. All of these systems are relatively technically complicated, and for most, their functionality in a living body is jeopardized by processes such as the growth of cells onto sensor components. Doppler sonographic measuring devices are also energy-intensive, and for this reason alone are not well suited for operation in the implanted state.
Using indirect methods, conclusions may also be drawn regarding blood movement in a living organism by use of pressure sensors, or other sensors which detect parameters that are a function of the blood flow. These include so-called hemodynamic sensors (HDS), or approaches which include intracardiac electrograms (IEGM). Such approaches are often relatively inaccurate and susceptible to malfunction.
A device is known from S. Gawad: “Micromachined impedance spectroscopy flow cytometer for cell analysis and particle sizing,” Lab on a Chip, 2001, 1, 76-82, for classifying and sorting cells or particles in a liquid stream on the basis of their size. The device is used as a microelectrode arrangement which introduces electrical excitation energy into a liquid tube and detects a resulting impedance signal which is characteristic of the cells or particles flowing through.
Systems for measuring the blood flow in the heart or in blood vessels are also known which use two respective interspaced electrodes on an electrode line and a catheter, one of the electrodes being designed as a polarizable cathode, for simultaneously producing and detecting a galvanic cell potential which in principle is a function of the flow rate of the blood into the surroundings of the electrodes. U.S. Pat. No. 5,799,350 describes a corresponding system which may be inserted into the heart for measuring the blood flow rate in the vicinity of the cardiac valves. A similar measuring system, used as measurement electrodes for the stimulation electrodes of a pacemaker lead, is also known from U.S. Pat. No. 5,602,342. A similar measuring system which, as with the previously described system, may be used in a cardiac stimulator, is disclosed in U.S. Pat. No. 7,011,633 B2. This system is used as a measuring electrode for a ring-shaped, cylindrical, or spirally wound flat electrode whose inner surface is used as an active measuring electrode surface.
The latter-referenced systems have not become widely established in pacemaker treatment, probably as a result of their insufficient measurement accuracy.